HOW TO FIND OUT WHY MY CHECK ENGINE LIGHT IS ON?

2003 Mercury Marauder • 90,100 miles

Could a bad computer to car make my car drive in 2nd gear and have the check light on? Took car to auto zone and the computer said it was a shift solenoid problem.

Took my car to a transmission shop and they check out my car for a day, they said transmission looked fine and cant find the problem and sometime the computer used to check light isnt always right. They said it might b that the computer to my car was bad and it might be electrical prob.

He said cars driving in 2nd gear. So is it possible the computer is bad and making my car drive this way